1. Field of the Invention
The present invention relates to an optical information recording medium for recording and reproducing information by using optical means such as a laser beam, and a method for manufacturing the medium.
2. Description of the Prior Art
There is conventional technology available for reproducing or recording information at high density by utilizing a laser beam. This conventional technology is commercialized mainly as optical disks.
Optical disks can be divided roughly into the read-only type, the write-once read-many type and the rewritable type.
The read-only type is commercialized as compact discs or laser disks, and the write-once read-many type and the rewritable type are commercialized as disks for recording document files and data files. As the rewritable type optical disk, a magneto optical disk and a phase-change type optical disk are mainly known.
The phase change type optical disk utilizes the reversible change of state of a recording layer between amorphous and crystalline states (or between a crystalline state and another crystalline state having a different structure) by irradiation with a laser beam. More specifically, when a laser beam is irradiated onto a phase-change type optical disk, at least one of a refractive index and an attenuation coefficient of a thin film for recording changes. In addition, when a laser beam is irradiated onto a recorded part of the phase-change type optical disk, the amplitude of light that passes through the recorded part, or light that is reflected by the recorded part, changes. As a result, the amount of the passed light or the reflected light that reaches the detection system changes so that the signal can be reproduced.
Generally in a phase-change type optical disk, the crystalline state of a recording layer material is used as the unrecorded state, while the amorphous state of the recording layer material is used as the recorded state. Here, the amorphous state is obtained by irradiating the laser beam so as to melt the recording layer material and by cooling the recording layer material rapidly. In addition, when erasing the signal, a laser beam of a smaller power than the power for recording is irradiated so that the recording layer becomes crystalline.
A chalcogen compound is often used as the recording layer material. The recording layer made of a chalcogen compound is deposited in the amorphous state, so it is necessary to crystallize the entire recording area in advance to give it the unrecorded state. This crystallization of the entire area is called “initialization”.
The initialization process is included in a process for manufacturing the disk, and a laser beam or a flash light source is used to crystallize the recording layer. When using a laser beam, the disk is rotated while the laser beam is irradiated and focused onto an information layer. Then, the position of the optical head is shifted in the radial direction of the disk, so that the entire surface of the disk can be initialized.
Initialization conditions including a laser power, a linear speed, a defocus amount and a feed pitch in this initialization are determined so as to satisfy the following criterion. Namely, the initialization conditions are generally determined so that the entire initialization area is crystallized uniformly without remaining in the amorphous state, and so that the quality of signal is constant during the period from the first recording to after a plurality of times (a few tens of times) of overwriting information.
In order to determine the initialization conditions, the initialization conditions are detected at a middle position in the radial direction of the disk (a predetermined position within a data area for recording and reproducing information). Furthermore, the detected initialization conditions are used for initializing the entire surface of the disk, including a burst cutting area that will be described later. Namely, the same conditions are used for initializing the entire surface of the disk.
Note that a single-sided two-layer structure and a method for manufacturing the structure have been proposed for increasing the recording capacity per unit area of an optical disk, and a technique for changing the initialization conditions for each of the layers is known (for example, see Japanese unexamined patent publication No. 10-132982). In addition, there is a document that describes a method for manufacturing a phase-change optical disk that has the single-sided two-layer structure and is used with a blue violet laser beam (for example, see Japanese unexamined patent publication No. 2000-400442).
The standards for a DVD-RAM and a Blu-ray disk describe that a burst cutting area (hereinafter referred to as “BCA”) is provided to a disk for identifying each disk.
This BCA is formed by cutting the film using a high power laser beam, or in a normal initialization process by providing initialized portions and uninitialized portions in the radial direction like a bar code (see FIG. 1), for example.
When forming the BCA in the initialization process, a relative angular velocity between the optical head of the laser beam for initialization and the disk is maintained at a constant value while the laser beam is turned on and off so that the initialized portions and the uninitialized portions are provided.
In addition, when reading information of the BCA, the disk is rotated, and the laser beam is focused onto a BCA region where the BCA is formed. Thus, the information of the BCA is read from the difference in reflectivity between a portion with the film and a portion without the film, or between the initialized portion and the uninitialized portion. Note that standards for a DVD-RAM and a Blu-ray disk describe that the BCA is provided to the innermost circumference on the rearmost layer viewed from the laser beam incident side.
The present inventors have been developing an optical disk having a phase-change type single-sided multi-layered structure using a blue-violet color laser beam. For example, they have been developing an optical disk including a transparent substrate on which a first information layer, an optical separation layer, a second information layer and a transparent layer are formed, as shown in FIG. 1. In addition, this optical disk has the BCA that is formed on the first information layer in the initialization process.
According to a study by the inventors, two problems are found in this initialization process as follows.
The first problem is that the second information layer may be initialized in part in the process for providing the BCA on the first information layer, when performing the initialization process on the first information layer and the second information layer one by one in this order.
The second problem is that a defocus may occur at the same radial area as the BCA region on the first information layer so that the initialization process is stopped during the initialization process of the second information layer after the initialization of the first information layer.